In recent years, development of displays has been remarkable. Liquid crystal displays (LCD), electroluminescence displays (EL), field emission displays (FED), or the like are used as display devices for business machines such as personal computers and word processors, and control systems in factories. These displays have a sandwich structure in which a display device is held between transparent conductive oxides.
A mainstream material for such a transparent conductive oxide is indium tin oxide (hereinafter occasionally abbreviated as “ITO”) prepared by a sputtering method, an ion plating method, or a vapor deposition method as described in Non-Patent Document 1.
ITO is composed of a specific amount of indium oxide and tin oxide, possesses excellent transparency and conductivity, can be etched using a strong acid, and exhibits high adhesion to a substrate.
Although ITO has excellent properties as a material for transparent conductive oxide, ITO is not only a scarce resource, but also contains a large amount (about 90 atomic percent) of indium which is a biologically harmful element. Moreover, the indium itself produces nodules (projections) during sputtering. The nodules produced on the target surface have been one of the causes of abnormal electrical discharge. In particular, when an amorphous ITO film is produced for improving etching properties, the indium compound on the surface of the target is reduced due to introduction of a small amount of water and hydrogen gas in the sputtering chamber, giving rise to further production of nodules. If an abnormal electrical discharge occurs, scattered materials become attached to the transparent conductive oxide as impurities during or immediately after the film formation.
The indium content in ITO must be reduced due to these problems of instability of supply (scarcity), hazardous properties and occurrence of nodules. However, if the content of indium in ITO is reduced to 90 atomic percent or less, a high-resistance tin compound in the target has electrical carriers (charges), leading to occurrence of abnormal electrical discharge.
In general, different etching solutions are used for etching a metal or a metal alloy and for etching ITO. Specifically, a phosphoric acid-based etching solution is used for etching a metal or a metal alloy (e.g. Al or Al alloy) and an oxalic acid-based etching solution is used for etching ITO. Therefore, productivity cannot be improved since equipment for an etching solution, such as a tank, is required to be provided separately for each material to be etched, and a wire and an electrode cannot be etched simultaneously.
As a method for preventing generation of nodules and suppressing abnormal electrical discharge, a hexagonal layered compound of In2O3(ZnO)m, wherein m is an integer of 2 to 20, with a crystal grain diameter of 5 μm or less has been investigated (Patent Documents 1 and 2). However, if the indium content is reduced to 70 atomic percent or less in this method, a hexagonal layered compound having a high resistance with an increased number of ZnO (In2O3(ZnO)m, wherein m is 4 or more) may be generated. Further, there are problems such as decrease of the sintered density and conductivity of the target, which causes abnormal electrical discharge and retards the film forming speed; low target strength, leading to easy cracking; and poor heat resistance in the presence of air of the transparent conductive film formed by sputtering.
Patent Document 1: WO01/038599
Patent. Document 2: JP-A-06-234565
Non-Patent Document 1:“Technology of Transparent Conductive Film” edited by The 166th Committee of
Transparent Oxide and Photoelectron Material, Japan Society for Promotion of Science, Ohmsha, Ltd. (1999)
The invention provides a target having a low resistance, high theoretical relative density and high strength, with a reduced indium content, which enables stable sputtering while suppressing abnormal electrical discharge generated when forming a transparent conductive film by sputtering, and is capable of forming a transparent conductive film which can be etched with a phosphoric acid-based etching solution which is an etching solution for a metal or an alloy, as well as a transparent conductive film prepared using such a target, a transparent electrode and a method for producing a transparent electrode.